The present invention relates to an optical system of an optical pick-up and an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc complying with a predetermined standard. More specifically, the present invention relates to optical system of an optical pick-up having an objective lens made of resin (hereafter, frequently referred to as a resin lens) having a numerical aperture larger than 0.8 which is suitable for information recording/reproducing for a high-recording density optical disc, such as BD (Blu-ray Disc), and an optical information recording/reproducing apparatus on which the optical system of an optical pick-up is installed.
There exist various standards of optical discs, such as CD (Compact Disc) and DVD (Digital Versatile Disc), differing in recording density, protective layer thickness, etc. Meanwhile, high-recording density optical discs (e.g., BD), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording.
Since the high-recording density optical disc has the higher recording density than DVD and CD, a beam spot having a relatively small diameter is required for information recording and reproducing for the high-recording density optical disc. That is, a large NA is required for information recording/reproducing (i.e., information recording and/or information reproducing) for the high-recording density optical disc. For this reason, in general a glass lens having a relatively high refractive index is used as an objective lens for the optical information recording/reproducing apparatus. However, a glass lens has a drawback that since a glass lens is heavy in weight, the glass lens places a burden on operation of an actuator for driving the lens when the actuator moves the lens in a focusing direction or in a tracking direction.
Therefore, a designer might consider forming a lens with resin in place of glass as material in order to reduce weight of the lens. By achieving the weight reduction through use of resin as material of the lens, it becomes possible to reduce the burden to be placed on the actuator for driving the objective lens. Furthermore, forming the objective lens for the high-recording density optical disc with resin provides advantages in regard to easiness of production, a high degree of productivity and cost. However, when resin is used as material of the lens, a drawback arises as described below. Each of Japanese Patent Provisional Publications No. 2008-176916A (hereafter, referred to as JP2008-176916A) and No. 2009-117030A (hereafter, referred to as JP2009-117030A) and Japanese Patent Publication No. 4193914 (hereafter, referred to as JP4193914) discloses an optical information recording/reproducing apparatus employing a resin objective lens. The optical information recording/reproducing apparatus disclosed in each of these publications has a configuration to prevent deterioration of optical performance caused by the drawback regarding the resin lens as described below.
In regard to a rate of change of refractive index caused when temperature changes, resin has the rate of change of refractive index which is more than ten times larger than the rate of change of refractive index of glass. Therefore, the resin lens has a drawback that the changing amount of spherical aberration due to change of refractive index (i.e., temperature changes) is considerably larger than that of a glass lens. For this reason, the optical information recording/reproducing apparatus disclosed in JP2008-176916A is configured such that a coupling lens is arranged to be movable to cancel the spherical aberration caused by the temperature changes by controlling the degree of divergence of a light beam incident on the objective lens. However, a temperature compensation mechanism of this type has a drawback that a dedicated component, such as a temperature sensor, is required and therefore the configuration of the apparatus becomes complicated.
The spherical aberration is also caused by the difference in protective layer thickness or in intermediate layer thickness of an optical disc. In general, the spherical aberration of this type is also corrected by using a coupling lens arranged to be movable. However, in order to correct both of the spherical aberration due to the temperature changes and the spherical aberration due to the difference in protective layer thickness or in intermediate layer thickness simultaneously, the coupling lens needs to have a wide movable range. In this case, it is required to increase the size of the optical information recording/reproducing apparatus. Furthermore, a drawback arises that a load placed on an actuator for moving the coupling lens becomes large. For this reason, in order to omit the temperature compensation mechanism, the optical information recording/reproducing apparatus disclosed in JP2009-117030A is configured such that the change of spherical aberration due to the temperature changes is suppressed by setting a focal length of an objective lens to be small. However, in this case, it becomes difficult to secure an adequate working distance, i.e., a distance between the protective layer surface of the optical disc and the surface of the objective lens facing the protective layer surface.
In order to omit the temperature compensation mechanism, the optical information recording/reproducing apparatus disclosed in JP4193914 is configured to give a lens surface shape for suppressing the temperature change rate of the 3rd-order spherical aberration to the objective lens. However, a diffraction structure formed on the lens surface is configured to produce high-order diffracted light, and therefore each annular zone has a great depth. In this case, a transferring failure may be caused during a molding process of the objective lens. When such a transferring failure occurs, lowering of the light amount is caused in the optical information recording/reproducing apparatus.